The invention relates to radio systems that apply frequency hopping (FH) spread spectrum techniques. More particularly, the invention relates to power control techniques for use in radio systems in which several, uncoordinated and nonsynchronized FH systems cover the same area.
In the past few decades, progress in radio and Very Large Scale Integration (VLSI) technology has fostered widespread use of radio communications in consumer applications. Portable devices, such as mobile radios, can now be produced having acceptable cost, size and power consumption characteristics.
Although wireless technology is today focused mainly on voice communications (e.g., with respect to handheld radios), this field will likely expand in the near future to provide greater information flow to and from other types of nomadic devices and fixed devices. More specifically, it is likely that further advances in technology will provide very inexpensive radio equipment that can be easily integrated into many devices. This will reduce the number of cables currently used. For example, radio communication can eliminate or reduce the number of cables used to connect master devices with their respective peripherals.
The aforementioned radio communications will require an unlicenced band with sufficient capacity to allow for high data rate transmissions. A suitable band is the Industrial, Scientific and Medical (ISM) band at 2.4 GHz, which is globally available. The band provides 83.5 MHZ of radio spectrum.
To allow different radio networks to share the same radio medium without coordination, signal spreading is usually applied. In fact, the FCC in the United States currently requires radio equipment operating in the 2.4 GHz band to apply some form of spreading when the transmit power exceeds about 0 dBm. Spreading can be either at the symbol level by applying direct-sequence spread spectrum or at the channel level by applying frequency hopping (FH) spread spectrum. The latter is attractive for the radio applications mentioned above since it more readily allows the use of cost-effective radios.
The range of a radio link is generally determined by the transmit power of the sender in conjunction with the receiver sensitivity of the recipient, the receiver sensitivity being that received signal level for which acceptable reception is just possible. The receiver sensitivity is normally determined by the noise characteristics in the receiver which in turn depend on the bandwidth and allowable supply currents. Generally, the receiver sensitivity of a radio is fixed at the time of manufacturing. In contrast, the transmit (TX) power is usually a variable. Apart from hardware and power supply limitations, the maximum TX power is limited by government regulations. Even in an unlicenced band like the 2.4 GHz ISM band, maximum TX power is limited to 1 W. However, in the type of applications mentioned above, it is unnecessary to fix the TX power at it maximum. Rather, the TX power is regulated down such that the recipient receives a just sufficient amount of signal power for acceptable link quality. Reducing the TX power to the level just needed will reduce power consumption, thereby not only extending battery life, but also reducing interference. Reduction of interference is especially important if many uncoordinated radio networks share the same medium.
The TX power should always be controlled to an acceptable minimum in order to maintain acceptable link quality. In the type of applications mentioned above, the communicating radio units are peer units, and each seeks to reduce its TX power as much as possible. This results in a closed-loop power control algorithm, in which the recipient informs the sender to increase or decrease its TX power depending on the receive conditions. Such an automatic power control scheme has been described by G. H. Flammer, in U.S. Pat. No. 5,465,398, issued Nov. 7, 1995 (xe2x80x9cAutomatic Power Level Control of a Packet Communication Linkxe2x80x9d). This patent describes a procedure in which the TX power of the sender is regulated based on Received Signal Strength Indication (RSSI) in the recipient. In accordance with the described conventional technique, power control is relative in that the lowest RSSI value of a successfully-received packet is used as a reference value. xe2x80x9cSuccessfulxe2x80x9d in this context means that the entire packet, including the payload data, has been received without error. For those packets that are (successfully) received with a higher RSSI level, the difference between the higher RSSI level and the reference value is determined and communicated to the sender, which can then reduce its TX power. Packets that are not successfully received are retransmitted at a higher TX power.
The problem with this scheme is that it does not distinguish between range and interference. The failure to successfully receive a packet can be attributed either to the signal level being too low, or to the interference level being too high. This is especially true in a situation in which many uncoordinated radio systems cover the same area: these systems will interfere with each other and packets will be lost due to collisions of different radio transmissions. Were an automatic power control strategy such as that described by Flammer to be employed under these conditions, all radio units would increase their power, which would only worsen the situation because the coverage area and therefore the number of mutual interferers would increase. In fact, the systems may become unstable. In an unlicenced band like the ISM band where operation of the radio units is uncoordinated and the radio units operate independently of each other, a power control strategy based on interference will result in an unfair domination of that system having the largest TX power.
An additional problem relates to the bursty interference conditions in FH systems: as the different systems hop uncoordinated through the spectrum, the interference only occurs if they happen, by chance, to use the same hop frequency at the same time. Due to the hopping, the interference conditions change for every hop. If the system hops at the packet rate, adjusting the power based on the successful reception of a packet is not very stable.
The foregoing and other objects are achieved in transmission power control methods and apparatuses for use in a frequency-hopping radio system that transmits packets from a sending radio unit to a receiving radio unit, wherein each packet includes an address designating the receiving radio unit. In accordance with one aspect of the invention, the received signal strength of packets whose addresses were successfully received in the receiving radio unit is measured, regardless of whether other portions of the respective packets were successfully received; and an average signal strength value is generated from the received signal strength measurements. A mathematical difference between the average signal strength value and a target value associated with the receiving radio unit is then determined, and used as a basis for deciding whether to send a power control message from the receiving radio unit to the sending radio unit.
In another aspect of the invention, using the mathematical difference as a basis for deciding whether to send a power control message from the receiving radio unit to the sending radio unit comprises sending a power control message from the receiving radio unit to the sending radio unit if the mathematical difference is greater than a first decision boundary or less than a second decision boundary.
In yet another aspect of the invention, the power control message may include the mathematical difference.
In still another aspect of the invention, the power control message is received in the sending radio unit, which then adjusts its transmission power level unit by an amount based on the mathematical difference.
In yet another aspect of the invention, adjusting the transmission power level in the sending radio unit by an amount based on the mathematical difference includes determining whether the amount based on the mathematical difference would cause an adjusted transmission power level to exceed a predefined maximum transmission power level. If the amount based on the mathematical difference would cause the adjusted transmission power level to exceed the predefined maximum transmission power level, then the transmission power level in the sending radio unit is adjusted to be no more than the predefined maximum transmission power level.
In still another aspect of the invention, when the sending radio unit is at the predefined maximum TX power level, a control message is sent from the sending radio unit to the receiving radio unit informing that a maximum transmission power level has been reached.
In yet another aspect of the invention, the receiving radio unit responds to the control message from the sending radio unit informing that a maximum transmission power level has been reached, by sending no further power control messages to the sending radio unit that instruct the sending radio unit to further increase its transmission power level.
In still another aspect of the invention, adjusting the transmission power level in the sending radio unit by an amount based on the mathematical difference includes determining whether the amount based on the mathematical difference would cause an adjusted transmission power level to fall below a predefined minimum transmission power level. If the amount based on the mathematical difference would cause the adjusted transmission power level to fall below the predefined minimum transmission power level, then the transmission power level in the sending radio unit is adjusted to be no less than the predefined minimum transmission power level.
In yet another aspect of the invention, when the sending radio unit is at the predefined minimum TX power level, a control message is sent from the sending radio unit to the receiving radio unit informing that the minimum transmission power level has been reached.
In still another aspect of the invention, the receiving radio unit responds to the control message from the sending radio unit informing that a minimum transmission power level has been reached, by sending no further power control messages to the sending radio unit that instruct the sending radio unit to further decrease its transmission power level.
In yet another aspect of the invention, the target value associated with the receiving radio unit is based on the receiver sensitivity alone, or adjusted to account for implementation losses and other inaccuracies.
In still another aspect of the invention, generating the average signal strength value from the received signal strength measurements may include averaging signal strength values from the received signal strength measurements over a period of time extending over at least two frequency hops.
In yet another aspect of the invention, the power control message is transmitted on a control channel established between the receiving radio unit and the sending radio unit. Alternatively, it may be included in a return packet that is transmitted from the receiving radio unit to the sending radio unit.
In still another aspect of the invention, a highest permissible transmit power level is always used to send the power control message from the receiving radio unit to the sending radio unit. Alternatively, a first transmit power level is initially used to send the power control message from the receiving radio unit to the sending radio unit. The power control message transmit power level is then gradually increased from the first transmit power level to successively higher levels until a reception signal strength level at the receiving radio unit has reached a predefined acceptable level.